1. Field of the Invention
This invention relates to a method for preparing blackened steel strips or sheets having a blackened layer of even outer appearance for use in buildings, electric appliances and automobiles.
2. Prior Art
At present, steel strips having a blackened surface are widely used in electric appliances, copying machines, information communications machines, automotive parts, building interior members and the like. Such blackened steel strips are manufactured by various techniques including (1) black paint coating, (2) chemical treatment, (3) blackening chromate treatment, (4) anodic treatment, and (5) cathodic treatment. Most industrial among others is the anodic treatment as disclosed in Japanese Patent Publication (JP-B) Nos. 60915/1986, 46158/1988, and 46159/1988.
The anodic treatment is to accomplish coloring by plating a zinc alloy layer on a steel strip and dissolving some of the alloy components from the plating layer into an electrolytic solution. The anodic treatment is thus industrially disadvantageous in this regard as compared with the cathodic treatment, but has advantages of quality including tight adhesion and uniform outer appearance over the remaining techniques. We previously proposed in Japanese Patent Application Kokai (JP-A) No. 143293/1992 a new method for preparing a colored steel strip in order to produce a blackened steel strip having a uniform color layer independent of the underlying plating layer.
Starting production of colored steel strips on a commercial scale, we encountered several problems including (1) generation of metal hydroxides such as zinc hydroxide during the process, (2) dropping of any deposit from the cathode, and (3) occurrence of spark marks.
As the production quantity increases, zinc accumulates in the electrolytic solution due to anodic treatment. Since the electrolytic solution is increased in pH near the cathode due to hydrogen generation, pH adjustment is necessary on the cathode side. In practice, the electrolytic solution is pH adjusted when it is fed from the electrolytic tank back to the circulation tank. Due to this time lag, the solution in the electrolytic tank is at higher pH than in the circulation tank. Then, as zinc accumulates, zinc forms a hydroxide due to a pH rise in the electrolytic tank. This zinc hydroxide does not disappear, but is suspended in the solution and deposits on the conveyor rolls where such deposits can be caught by the strip moving along the rolls, causing defects and damages to the strip. At the same time, alloy components adhering to the cathode without being completely reduced to metallic state can be stripped therefrom and deposited on the conveyor rolls where such deposits cause damage to the steel strip.
A horizontal electrolytic tank is designed such that electricity is conducted to a steel strip via conductor rolls located at the inlet and outlet. However, if electric conduction is performed at both the inlet and outlet during blackening process, small defects having metallic luster are formed on the strip surface near the outlet due to spark marks. If whole electric current is introduced at the inlet conductor roll, Joule's heat generates to heat the steel strip especially when it is thin and at the same time, increases the temperature of the electrolytic solution, adversely affecting the black outer appearance of the strip. In order to continue production while keeping the black outer appearance satisfactory, the production rate must be reduced to one-half of the system capacity.
To ensure consistent production of blackened steel strips of quality, there are employed countermeasures that (1) the electrolytic solution is replaced by a fresh one when the zinc concentration in the solution exceeds a predetermined level, (2) the electrode is taken out, polished and cleaned after a predetermined quantity of steel is treated, and (3) operation is continued at a production rate reduced to one-half of the system capacity. These countermeasures have the drawbacks of an increased downtime, reduced operation rate, and reduced production efficiency.
Such problems might be overcome by providing (1) zinc removal means or (2) an additional electrolytic tank at the sacrifice of economy. That is, capital investment is increased. The latter (2), which is proposed in JP-A 17695/1992, is difficult in maintenance and control since acid or alkali is sprayed to the conduction rolls to indirectly accomplish surface treatment of a steel strip. The strip is liable to variations in a transverse direction due to decoloring.
As previously described, generation of hydrogen gas on the cathode side causes the electrolytic solution to increase its pH so that zinc being dissolved out may form a hydroxide which is suspended in the electrolytic solution and seized by the rolls as foreign matter, causing defects. Such problems may be solved by lowering the pH of the electrolytic solution. However, since a pH lowering allows for etching, electrolysis at such low pH results in a steel strip having increased surface roughness and losing luster. Etching also increases factors of detracting from outer appearance as by removing some of the blackened layer to cause decoloring thereof and incurring non-uniform outer appearance in a transverse direction. It was difficult to solve these problems.